.Call her and apologize. Tell her you were babysitting your seven-year old, nephew who was using your phone to play games, while you were outside doing yard work, or working in the other room. You didn't realize she had called and that your nephew answered the phone. Thinking he was being funny, he farted into the phone.

You just found out about it from your sister after she overheard her son telling a friend of his. You looked at your call log and realized, to your horror, the call your nephew answered was from the company. Apologize and tell her you're really embarrassed.

Most people can surely relate to kids doing embarrassing things.

This sounds very plausible.